The present invention relates to an eddy current testing apparatus and an eddy current testing method, and more particularly, to an eddy current testing apparatus and an eddy current testing method preferably applicable to inspect internal surfaces of fork pin holes formed in blade forks for turbine blades attached to the rotor of a steam turbine.
To facilitate the manufacturing and maintenance of a steam turbine used in a power generating plant, its rotational axis and turbine blades are manufactured separately and then assembled. Specifically, blade fork portions formed at the roots of the turbine blades are assembled to disc fork portions formed on a disc provided on the rotational axis of the steam turbine, and the turbine blades are fixed to the disc by inserting pins into fork pin holes formed in these fork portions (see FIG. 2 in Japanese Patent Laid-open No. 2001-12208). As the steam turbine rotates, stress is applied to the structural material of the turbine blade in the vicinity of the fork pin holes in the blade fork portion. Accordingly, cracks may be generated in the vicinity of the fork pin holes in the turbine blade.
Conventional inspection for cracks in the vicinity of the fork pin holes in the turbine blade is performed by removing the turbine blade from the disc and then applying magnetic particle testing (hereinafter referred to as MT). The MT is a method for detecting a leak of a magnetic flux from a defect in a test object when a magnetic field is applied to the test object. This MT is applied to the turbine blade, which is a test object, as described below. While a magnetic field is applied to the blade fork portion of the turbine blade, the surface of the blade fork portion is coated with magnetic metal particles on which a fluorescent material is applied, the fluorescent material being accumulated by the magnetic flux leaking from a defect. Ultraviolet rays are then irradiated to the blade fork portion. The magnetic metal particles accumulate in, for example, a defect in a fork pin hole. Since ultraviolet rays are irradiated, whether the magnetic metal particles accumulate can be determined by observing whether there is fluorescent light, and thus whether there is a defect can be determined. In inspection for defects on the basis of the MT, it is necessary not only to remove pins by which a disc fork portion of a disc and a blade fork portion of a turbine blade are joined but also to remove the turbine blade from the disc. When the turbine blade is inspected for defects on the basis of the MT, therefore, it takes much time. Upon the completion of the inspection, an additional task is needed to fit the turbine blade into the disc and combine them with pins.
Eddy current testing (ECT) is often used as a method of inspecting the surface of a test object for cracks. Particularly, the ECT is widely used to inspect the internal surfaces of tubular objects such as heat transfer tubes because an ECT probe can be moved quickly in the tube and thus advantages of functions suitable for high-speed ECT inspection can be fully taken. An example of ECT is described in Patent Laid-open No. Hei 8 (1996)-145954. In the ECT disclosed in Japanese Patent Laid-open No. Hei 8 (1996)-145954, a test probe including an ECT sensor is inserted into a tube to check whether the thickness of the tube is thinned and the interior is corroded.